Electric assist steering system are well known in the art. Electric power assist steering systems that utilize a rack and pinion gear set provide power assist by using an electric motor to either (i) apply rotary force to a steering input shaft connected to a pinion gear, or (ii) apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (i) a driver's applied torque to the vehicle steering wheel, and (ii) sensed vehicle speed.
In U.S. Pat. No. 3,983,953, an electric motor is coupled to the input steering shaft and energized in response to the torque applied to the steering wheel by the vehicle operator. An electronic control system includes a torque sensor and a vehicle speed sensor. A computer receives the output signals provided by both sensors. The computer controls the amount of the assistance provided by the motor dependent upon the applied steering torque and the sensed vehicle speed.
U.S. Pat. No. 4,415,054 to Drutchas (now U.S. Reissue U.S. Pat. No. 32,222,), assigned to TRW Inc., includes a D.C. electric assist motor driven through an H-bridge arrangement. The motor includes a rotatable armature encircling a steering member which has a thread convolution portion and a portion having straight cut rack teeth thereon. Rotation of the electric assist motor armature causes linear movement of the steering member through a ball-nut drive arrangement drivably coupled to the thread convolution portion of the steering member. A torque sensing device is coupled to the steering column to sense driver applied input torque to the steering wheel. The torque sensing device uses a magnet Hall-effect sensor arrangement for sensing relative rotation between the input and output shafts across a torsion bar. An electronic control unit ("ECU") monitors the signal from the torque sensing device and controls the electric assist motor in response thereto. A vehicle speed sensor provides a signal to the ECU indicative of the vehicle speed. The ECU controls current through the electric assist motor in response to both the sensed vehicle speed and the sensed torque demand signal. The ECU decreases steering assist as vehicle speed increases. This is commonly referred to in the art as speed proportional steering.
U.S. Pat. No. 4,660,671 discloses an electric controlled steering system that is based on the Drutchas steering gear. In the '671 arrangement, the D.C. motor is axially spaced from the ball-nut and is operatively connected thereto through a connection tube. The electronic controls includes a plurality of diagnostic features that monitor the operation of the steering system. If an error in the operation of the electric steering system is detected, the power assist system is disabled and steering reverts to an unassisted mode.
U.S. Pat. No. 4,794,997 to North, assigned to TRW Cam Gears Limited, discloses an electric assist steering system having an electric motor operatively connected to the rack through a ball nut. A vehicle speed sensor and an applied steering torque sensor are operatively connected to an ECU. The ECU controls electric current through the motor as a function of both applied steering torque and sensed vehicle speed. The current is controlled by controlling the percentage of the pulse-width-modulated ("PWM") signal applied to the motor. As the PWM increases, power assist increases. The ECU or computer is preprogrammed with discrete control curves that provide steering assist values (PWM values), also referred to as torque-out values, as a function of applied steering torque, also referred to as torque-in values, for a plurality of predetermined discrete vehicle speed values. Each vehicle speed value as an associated torque-in vs. torque-out control curve.
FIG. 3 of the '997 patent shows torque-in vs. torque-out control curves stored in the ECU. There is a torque-in vs. torque-out curve used for low speed vehicle maneuvering such as vehicle parking. Also, there is a torque-in vs. torque-out curve used for high speed maneuvering. Each of these control curves permits maximum assist should the applied steering torque reach an associated value. For vehicle speeds between the minimum speed curve and the maximum speed curve, a plurality of discrete curves are provided. The other discrete vehicle speed curves are all between the low and high speed curves. From these torque-in vs. torque-out curves, it can be seen that power assist decreases as vehicle speed increases. The transition from one level of power assist to another level of power assist for the different vehicle speeds occurs in steps or jumps. Changes in assist level in this type of system can be felt by the vehicle operator when vehicle speed changes occur during a steering maneuver.